No AV software - free or commercial - are 100%. But more critical than that is how often they and you update it. The big commercial ones (ie Norton and Mcafee) do tend to more on the ball than the free ones, but don't let that put you off the free ones. Just remember, the definitions should be updated at least daily though.
Also remember, AV software will only stop viruses (and some trojans). It is not intended to stop other nasties, such as browser hijacks (massively on the increase again), ad tracking and targetted ads, password snoopers, keyloggers, and other malware.
For general malware, adaware is pretty good, though bloated now, though on my Vista PCs, I just stick to the included Windows Defender most of the time.
Not much to prevent browser hijacking, hijackthis can be useful to clean up some (but not all) hijacks, but needs skill and knowledge to use effectively.
Whenever any of my machines are compromised - which is rare - they get rebuilt. It is simply not possible to guarantee 100% cleanup.